1. Field of the Invention
The present invention relates to an optical receiving device, a waveform optimization method for optical data signals, and a waveform optimization program for optical data signals. More particularly, the present invention relates to an optical receiving device that is designed to minimize waveform deterioration in optical data signals, a waveform optimization method for optical data signals, and a waveform optimization program for optical data signals.
2. Description of the Related Art
Recent optical transmission systems incorporating optical amplifiers and the technology of WDM (wavelength division multiplexing) have more factors that may cause a deterioration in optical receive waveform than conventional optical transmission systems.
One example of factors that may cause a deterioration in optical receive waveform is the accumulation of noise, resulting from ASE (amplified spontaneous emission) from optical fiber amplifiers. Other examples are the dispersion and non-linear effects of optical fibers, whose influence has become more significant as optical signal power within optical fibers has increased. In WDM, such an increase in waveform deterioration is often caused by crosstalk interference from adjacent channels.
In the past, this problem was alleviated through provisions on the receiving device side by, for example, optimizing decision threshold positions in a CDR (clock and data recovery circuit). In order to further improve receiving sensitivity, however, these provisions are not sufficient, but the compensation of waveform deterioration itself must be addressed.
In an optical transmission system, the optical receive waveform of a signal has quite a large eye opening, which is an area where optical input data signals can be discriminated accurately between 1 and 0, immediately after the signal is sent out, as shown in FIG. 35. After a transmission over a distance of 600 km, however, the eye opening in the optical receive waveform is very small, as shown in FIG. 36.
The discrimination circuit of an optical receiving device is required to perform optimum discrimination in the presence of such deterioration in optical receive waveform. The optimum value for a decision threshold position, at which 1 or 0 is discriminated, also varies with light receiving power, because the eye opening in an optical receive waveform varies with light receiving power. As far as conventional optical transmission lines are concerned, the issue of the light receiving power is negligible in practical use. The decision threshold position can be chosen from a relatively broad range without causing the deterioration of optical receive waveform, and thus the decision threshold position could be fixed at any value pre-determined as appropriate by the manufacturer before shipment.
As described above, it is necessary to compensate waveform deterioration directly in order to improve receiving sensitivity in the presence of the waveform deterioration resulting from the dispersion of optical fibers, one of the factors for waveform deterioration.
Furthermore, for a typical optical transmission system incorporating an optical amplifier and WDM, it is difficult to maintain the quality of a transmission line by using a conventional discrimination circuit with a fixed decision threshold position. This is because the increased number of factors that are responsible for deterioration in optical receive waveform as described above causes a discrimination circuit to inherently suffer from a curve (floor) in error rate characteristic (see the upper diagram of FIG. 37).